Chemical shift anisotropy parameter

Four cationic antimalarials, chloroquine (1), quinacrine (2), mefloquine (3), and quinine (4), have been studied using DPPC bilayer membranes and 31P- and 2H-NMR. Figure 3.26 shows the 31P spectra of DPPC, fully hydrated above Tt, in the absence and presence of three of the drags. Mefloquine exerts the strongest effect on the chemical shift anisotropy parameter (most effective broadening of the foot ),... 

The process of spin-lattice relaxation involves the transfer of magnetization between the magnetic nuclei (spins) and their environment (the lattice). The rate at which this transfer of energy occurs is the spin-lattice relaxation-rate (/ , in s ). The inverse of this quantity is the spin-lattice relaxation-time (Ti, in s), which is the experimentally determinable parameter. In principle, this energy interchange can be mediated by several different mechanisms, including dipole-dipole interactions, chemical-shift anisotropy, and spin-rotation interactions. For protons, as will be seen later, the dominant relaxation-mechanism for energy transfer is usually the intramolecular dipole-dipole interaction. 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

Here, ojr is the rate of spinner rotation. I is the proton spin number, 8 is the chemical shift anisotropy (CSA) and q is the asymmetric parameter of the CSA tensor. Thus, the line broadening occurs when an incoherent fluctuation frequency is very close to the coherent amplitude of proton decoupling monotonously decreased values without such interference in Figure 1. 

Fig. 12 C -detected C CSA patterns of the SHPrP109 i22 fibril sample. The upper and lower traces correspond to the experimental and simulated spectra, respectively. Simulations correspond to the evolution of a one-spin system under the ROCSA sequence. The only variables are the chemical shift anisotropy and the asymmetry parameter. A Gaussian window function of 400 Hz was applied to the simulated spectmm before the Fourier transformation. (Figure and caption adapted from [164], Copyright (2007), with permission from Elsevier)...

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Because the atoms occupy highly symmetric lattice positions, anisotropic parameters such as chemical shift anisotropy (CSA) or nuclear quadrupole coupling constants (NQCC) are either zero or fairly small. 

It is worth mentioning that parameter p is insensitive, to first order approximation, to modulation of the residue-specific 15N chemical shift anisotropy tensor and/or dipolar interaction, as the (d2 + c2) term in the R) / R ratio is canceled out. The noncollinearity of the CSA and dipolar tensors will require corrections to Eqs. (10) and (12) for high degrees of rotational anisotropy (D /D > 1.5), as described in detail in Ref. [22]. 

J-splitting, when it exists, imposes the definition of new spin quantities. These quantities also evolve according to relaxation phenomena and may interfere (by relaxation) with the usual magnetization components. This latter interference stems precisely from cross-correlation rates, i.e., relaxation parameters which involve two different mechanisms, for instance the dipolar interaction and the so-called Chemical Shift Anisotropy (27,28) (csa)... 

Wu has also reported solid-state Zn NMR spectra for ZnO, ZnS, ZnS04 7H20 and Zn(CH3COO)2. From the Zn NMR spectra obtained for stationary and magic-angle spinning (MAS) powder samples, useful parameters of chemical shift anisotropy (CSA)... 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

As regards the phenyl ring motions, interesting features are obtained from measurements of 13C chemical shift anisotropy of protonated and unpro-tonated aromatic carbons. The chemical shift parameters are orthogonal to each other, with o and 072 in the phenyl plane and <733 bisecting the phenyl plane. 

C chemical shift anisotropies of the unsaturated carbons. The chemical shift parameters used in the Ar-Al-PA for the unprotonated and proto-nated aromatic carbons, as well as for the carbonyl groups, are shown in Fig. 80. 

RDCs belong to the so-called anisotropic NMR parameters which cannot be observed in isotropically averaged samples as, for example, is the case in liquids. Besides RDCs, a number of other anisotropic parameters can be used for structure elucidation, like residual chemical shift anisotropy, residual quad-rupolar couplings for spin-1 nuclei, or pseudo-contact shifts in paramagnetic samples. Here, we will focus on RDCs where we give a brief introduction into the dipolar interaction, then into the averaging effects with the description by the alignment tensor and concepts to deal with the flexibility of molecules. For the other anisotropic NMR parameters, we refer the reader to ref 19 for an introduction and to refs. 6-8 for a detailed description. 

Benzene was the first molecule studied by NMR within liquid crystals, that is oriented in nematic liquids.81 This opened up much research, using benzene,82 leading to information about the chemical-shift anisotropy and selected spin-spin couplings. Isotope substitution too played a major role for example see Ref. 83. The 1H NMR powder spectrum at ca. 225 K gave principal values of the proton-shift parameter matrix.84 Various isotopically labelled versions of benzene... 

The nitrogen-15 nmr spectra (Figure 3) of sodium exchanged Alaskan and HB33 mordenites were analyzed for the chemical shift anisotropy tensors. The chemical shift anisotropy tensors have been measured for solid nitrogen at 4.2K(23) and calculated to be 603 28 ppm for the static molecule. The observed chemical shift tensors can be used to calculate an orientational parameter , where... 

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